Semiconductor laser device with plural active layers and changing optical properties

ABSTRACT

A wavelength-tunable semiconductor laser device presenting a large wavelength-tunable range or a very-high-speed modulating semiconductor laser device having a distributed feedback structure including a diffraction grating as in the case of a DBR laser or a DFB laser incorporates therein a plurality of active layers differing from one another in constituent elements or composition ratio or thickness for reducing spectral line widths, while improving single-mode spectral oscillation characteristics.

This application is a divisional of 07/537,901 filed Jun. 13, 1990, nowU.S. Pat. No. 5,119,393.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a semiconductor device. Moreparticularly, the present invention is concerned with a semiconductordevice which can be advantageously and profitably applied to asingle-mode oscillation semiconductor device typified by awavelength-tunable semiconductor laser device for a coherent opticalcommunication, a semiconductor laser device for a high-speedcommunication and others.

2. Description of the Related Art

For a large capacity optical communication system the use of awavelength-tunable semiconductor laser device and a modulatingsemiconductor laser device is required. For a better understanding ofthe present invention, description will first be made in some detail ofthe technology known heretofore in conjunction with these laser devices.

First, the wavelength-tunable semiconductor laser is considered. As oneof the large-capacity communication systems, there can be mentioned acoherent optical communication system in which an interference receiveris employed not only for making it possible to increase the receptionsensitivity but also for allowing only light signals of differentwavelengths to be selectively extracted by varying the wavelength of theinterference light for reception. In other words, with the coherentoptical communication system, there can be realized awavelength-multiplexed communication in which channel selection can beeffected with the aid of the light wavelength by employing a laser lightsource having a changeable oscillation wavelength in the receiver.Intrinsically, the optical communication has a significantly increasedtransmission capacity because of the capability of high-speed modulationas compared with the electronic signal transmissions utilizedheretofore. Further, in the case of the wavelength-multiplexedcommunication, it is possible to increase surprisingly the transmissioncapacity because a great number of light signals of differentwavelengths can be transmitted through the medium of only one opticalfiber. For these reasons, the coherent wavelength-multiplexed opticalcommunication is now attracting increasingly public attention as one ofthe communication techniques for supporting the age of large-capacitycommunication anticipated in the not too distant future.

In order to realize the wavelength-multiplexed optical communication,the receiver side a semiconductor laser is required to have as broad awavelength-tunable range or width as possible and capable of a singlewavelength oscillation (single-mode oscillation).

As the wavelength-tunable semiconductor laser reported in the past,there can be mentioned a laser described in "Electronics Letters", Vol.23, No. 8, (1987), pp. 403-405, the structure of which is shown in FIG.8 of the accompanying drawings. This semiconductor laser is referred toas the DBR laser (an abbreviation of distributed Bragg reflection laser)and composed of three regions provided on a substrate 806, i.e. adistributed Bragg reflection region (DBR region) 801, a phase controlregion 802 and an optical amplification region 803, wherein p-typeelectrodes 821, 822 and 823 are provided independently on theabove-mentioned regions, respectively, with a common n-type electrode824 being provided on the lower surface of the substrate 806. An activelayer 815 is formed solely in the optical amplification region 803,whereby light is amplified in the optical amplification region bycarriers injected through the electrode 823. The two other regionsinclude no active layer and are constituted with a passive opticalwaveguide 813 whose refractive index is forced to vary by the carriersinjected through the electrodes 821 and 822.

As is known in the art, the oscillation wavelength of the DBR laser isdetermined by the Bragg reflection wavelength which in turn isdetermined on one hand by a product (optical pitch) of the effectiverefractive index experienced by the light transferred through theoptical waveguide within the DBR region 801 and the pitch of adiffraction grating 812 implemented in the Bragg reflection region 801and on the other hand by a resonance wavelength satisfying the phasecondition of light traveling through the optical waveguide 813reciprocatively between an edge 820 located on the side of the opticalamplifier region and the DBR region. Consequently, in order to vary thelaser oscillation wavelength continuously, it is necessary to varysimultaneously both the Bragg reflection wavelength and the resonancewavelength while maintaining both of these wavelengths so as to coincidewith each other. According to the above-mentioned prior art technology,a continuous wavelength-tunable width or range is realized for a singlewavelength (in a single-oscillation mode) by controlling both the Braggreflection wavelength and the resonance wavelength.

In this conjunction, another example of the DBR laser is reported in"Applied Physics Letter", Vol. 52, No. 16, (1988), pp. 1285-1287. In thecase of this known wavelength-tunable semiconductor laser, the DBRregion is formed of an optically active material similar to the opticalamplification region, wherein independent electrodes are providedseparately for the purpose of allowing the oscillation wavelength tochange under the influence of the change in the density of injectedcarriers, as in the case of the preceding example. However, the DBRlaser now under consideration differs from that disclosed in the firstmentioned literature in that a greater change of the refractive indexascribable to an absorption edge shift effect which accompanies thecarrier injection is made available by using the optically activematerial for the DBR region. By virtue of this structure, a value aslarge as 11.6 nm is realized for the wavelength-tunable width. (It ishowever noted that the tunable width or range realized in this prior artDBR laser is not for the continuous change of wavelength but fordiscrete change thereof.)

The structure of the second mentioned DBR laser however suffers from theproblems in that absorption loss brought about by the free carriers isincreased as the amount of carrier injected increases, because a passivematerial is used for the DBR region and the phase control region,whereby the value or level of threshold current for oscillation of theoptical amplification region is increased, which results in lowering ofthe laser output power and increasing in the spectral line width. Inthis conjunction, it is to be noted that the coherent opticalcommunication requires a narrow spectral line width of the laser light.Accordingly, the phenomenon of increasing is brought about the spectralline width is brought about by changing the wavelength can never betolerated.

A semiconductor laser device developed in an effort to solve thetechnical problem of the increasing of the spectral line width isdisclosed, for example, in JP-A-64-49293 (Japanese Patent ApplicationLaid-Open No. 49293/1989). In this known semiconductor laser, anoptically active layer exhibiting a gain is provided in the DBR region.With this known semiconductor laser structure, it is certainly possibleto compensate for the absorption loss occurring upon current injectioninto the phase control region by the gain of the active layer providedin the DBR region, whereby the spectal line width can be suppressed fromincreasing. Further, such a structure is also known in which the phasecontrol region and the optical amplification region are finely dividedand arranged alternately with each other for thereby protecting thelaser characteristics against degradation due to the absorption lossmentioned above. In this conjunction, reference may also have to be madeto JP-A-64-14988.

Now, description will be turned to the semiconductor laser formodulation.

In the optical communication system, one of the requisite performancesimposed on the system is how densely the signal can be transmitted andreceived. To this end, high-speed response capability must be ensured inboth the light signal transmitter and receiver devices. In general,modulation of the semiconductor laser light with a high-speed currentpulse signal is ordinarily attended with fluctuation in the wavelengthdue to change in the refractive index internally of the laser. Thisphenomenon is known as wavelength chirping. Causes for wavelengthchirping will be explained below.

The density of carriers injected in the active region of thesemiconductor laser by current pulses of a modulation signal is forcedto change in accordance with the modulating signal. In this conjunction,it is noted that a phase delay occurs in the laser output light pulserelative to the change in the carrier density. As the consequence, thecarrier density becomes dominant such that the carriers exist in excesswhen compared with the stable state.

Upon oscillation of the laser light, the carrier density tends todecrease toward the stable state as a result of the stimulated emission.This means that during the oscillation of the laser light, the carrierdensity continues to change. Since the refractive index of the activelayer has a dependency on the carrier density, the change thereof bringsabout corresponding fluctuation in the laser oscillation wavelength.This is a cause of the wavelength chirping phenomenon. Since the opticalfiber employed for the optical communication or the like applicationexhibits a wavelength-dependent dispersion of the refractive index(wavelength dispersion), occurrence of the wavelength chirping givesrise to distortion in the pulse waveform. This provides a major factorfor limiting the distance of transmission in the high-speedtransmission.

With a view to reducing or suppressing the wavelength chirping takingplace upon high-speed modulation of the semiconductor laser, there hasalready been reported a method of applying electric currents to aplurality of electrodes disposed in cascade along the direction of theresonator. In this conjunction, reference may have to be made to "IEEEJournal of Lightwave Technology", LT-5, No. 4, (1987), pp. 516-522. Morespecifically, according to this known method, a plurality of electrodesare provided for the active regions having a same composition (samebandgap), wherein the mutually different electric currents are soapplied to the plural electrodes that the wavelength chirping can bereduced.

SUMMARY OF THE INVENTION

The wavelength-tunable laser implemented in such structure forsuppressing the spectral line width from increasing, as described above,suffers from a problem that it exhibits a high gain for the injectedcarriers, because the same active layer is employed for both the opticalamplification region and the DBR region. In other words, upon injectionof the carriers in the DBR region for changing the wavelength,self-oscillation takes place only in this DBR region to disadvantage.

This problem will be discussed below in more detail by reference to theaccompanying drawings. In case an active material is used for thediffraction grating region, the gain in this region becomes high andself-oscillation takes place at a gain coefficient which exceeds about40 cm⁻¹, whereby limitation is imposed to the change in the wavelength.Now, reference is made to FIGS. 15 and 16, wherein FIG. 15 is a view forgraphically illustrating changes in the refractive index of the opticalwaveguide and the gain brought about by the injection of carriers in thepassive optical waveguide. As the injection current I increases, therefractive index is decreased approximately in proportion to I^(1/2),which is also accompanied with increasing in the absorption, resultingin that the absorption loss is increased excessively to such an extentthat the spontaneous emission ceases. FIG. 16 of the accompanyingdrawings is a view for illustrating the change in the refractive indexin the case where an optical waveguide having a gain of a conventionalsemiconductor laser is employed in the diffraction grating region as anactive optical waveguide. For the carrier injection, there can berealized a greater rate of change of the refractive index as comparedwith that of the passive optical waveguide due to the band fillingeffect. However, the refractive index ceases to change at an increasedvalue of gain at which self-oscillation takes place.

Once the self-oscillation has taken place, the carrier density in therelevant region is fixed, resulting in that the range within which thewavelength can be tuned is limited, presenting a problem remaining to besolved that a desired wavelength-tunable width becomes unavailable.

Further, it is noted in conjunction with the DBR laser that thenecessity for the gain of the DBR region is only for the purpose ofcompensating for the absoption loss occurring in the phase controlregion. In other words, it is necessary to impart a sufficiently highgain to the DBR region for compensating for the absorption loss broughtabout by the current flowing through the phase control region.Accordingly, for a large magnitude of the absorption loss, the DBRregion starts to oscillate due to its own gain as a result of effort tocompensate for the absorption loss, thus giving rise to a basic problemto be solved. It should be added that the measures of dividing finelythe phase control region and the optical amplification region are not inthe position to eliminate drastically or effectively the absorptiontaking place in the phase control region.

On the other hand, in the case of the modulating laser in which themeasures for suppressing the wavelength chirping is adopted, asdescribed hereinbefore, there is a problem that the range for the biascondition and selection of the light output becomes necessarily limited,which is additionally attended with a problem that the high-speedcharacteristics are also limited.

It is therefore an object of the present invention to solve thetechnical problems of the prior art semiconductor laser described aboveand provide a semiconductor laser device which is capable of oscillatingstably at a desired wavelength.

Another object of the present invention is to provide a semiconductorlaser device capable of exhibiting an increased refractive index changewidth (or range) without being subjected to limitation imposed by theabsorption and the gain such as described above.

A still further object of the invention is to provide a semiconductorlaser device in which an increased wavelength-tunable width can berealized and fluctuation in the oscillation output at a selectedwavelength can be reduced by suppressing the gain coefficient fromincreasing due to carrier injection to a given one of a plurality ofactive layers.

In view of the above and other objects which will become more apparentas description proceeds, there is provided according to an aspect of theinvention a semiconductor laser device which comprises a plurality ofsemiconductor regions comprising active layers optically coupled to oneanother and susceptible to undergo changes of gain upon injection ofcarriers, the plurality of semiconductor regions including anamplification region comprising an optical amplification active layerfor emitting light in response to the injection of the carriers, a gainactive layer for guiding the light emitted by the optical amplificationactive layer, and a DBR region having a distribution feedback structurefor feeding back the light being guided, means for injecting thecarriers into the plurality of semiconductor regions, and a resonatorstructure for amplifying the light of a specific wavelength of thoseemitted by the optical amplification active region by selective feedbackthrough the feedback structure, wherein differential gain coefficient tothe injected carrier density of the gain active layer is made differentfrom the differential gain coefficient to the injected carrier densityin the optical amplification active layer.

With the phrase "active layer" used herein, it is intended to mean alayer exhibiting a gain greater than unity "1" (one), which in turnmeans that the layer is active to the light of a certain wavelength andthus exhibits an amplification function. Reversely, when a gain of alayer is not greater than "1", this means that the layer is passive, andmore specifically means that the intensity of light is constant withoutbeing caused to vary or that loss of light intensity is brought about byabsorption. The semiconductor laser device according to the inventionincludes a number of the active layers in the sense mentioned above.

It is one of the features characterizing the present invention that thedifferential gain coefficient to a density of carriers injected is madedifferent from one to another active layer. With the phrase"differential gain coefficient", it is intended to represent themagnitude of change in the gain to the magnitude of change in thedensity of carriers injected. By virtue of the different differentialgain coefficients imparted to the active layers, respectively, it ispossible to inhibit self-oscillation from occurring at least in one ofthe active layers. Difference in the differential gain coefficientbetween or among the active layers can be realized by using differentsemiconductor materials for forming the optically active layers or byvarying composition of the compound semiconductors constituting theactive layers or by varying the thickness of the active layers astypified by a quantum well structure. The difference in the differentialgain coefficient results in difference in energy between electron andhole which are combined to emit light, i.e. a bandgap or difference inthe energy state between the electrons and the holes within the activelayer forming the quantum well structure. In the active layer of a smalldifferential coefficient, the self-oscillation is suppressed. A regionin which such an active layer is formed is used to serve as the DBRregion.

In the semiconductor laser device according to the present invention,the plurality of active layers, i.e. the optical amplification layer andthe gain active layer are optically coupled directly or indirectlythrough the medium of other interposed (active or passive [having a gainnot greater than unity]) waveguide layer. Light emitted by the opticalamplification layer travels through the gain active layer to enter aresonator constituted through cooperation of the optical amplificationlayer and the gain active layer to be amplified within the resonator.The oscillation wavelength selected and amplified by the resonator isvariable by varying the effective resonator length. In view of the factthat the effective resonator length, i.e. optical path length has to beso established that the distance between edges of the resonator ordistance between the edge and a distributed feedback structure describedhereinafter satisfies the phase condition of light being transferred(traveling reciprocatively) within the resonator, it is preferred tomake variable the refractive index of the guide layer mentioned above.The region to serve for such phase control or adjustment is providedwith an electrode so that carriers can be injected into this regionindependently. It is not necessarily required to provide such phasecontrol region between the optical amplification active layer and thegain active layer. It is sufficient to implement the phase controlfunction in a region in which light travels within the resonatorstructure described above.

The semiconductor laser oscillation wavelength can be set or establishedby varying the refractive index internally of the resonator at a localregion which may be realized in a distributed feedback structureimplemented in a region where light is distributed. The distributedfeedback structure is generally constituted by a first semiconductorhaving a diffraction grating formed therein and a second semiconductorhaving a refractive index differing from that of the first semiconductorand stacked thereon so as to realize a periodic distribution of therefractive index.

According to another aspect of the invention, there is provided asemiconductor laser device which comprises a plurality of active layersexhibiting different gain peak wavelengths, and a resonator structurefor amplifying and oscillating light having a specific wavelengthdiffering from the gain peak wavelengths by selectively feeding back thelight of the specific wavelength. Thus, one of the featurescharacterizing the invention is seen in that the oscillation wavelengthis set with a deviation from the wavelength at which the gain of theactive layer is maximum (this wavelength is also referred to as the gainpeak wavelength).

According to a still further aspect of the invention, there is provideda semiconductor laser device which comprises a plurality of activeregions including a plurality of active layers coupled optically andhaving gains susceptible to change upon injection of carriers, means forinjecting the carriers into the plurality of the active regions, and aresonator structure for amplifying and oscillating light of a specificwavelength of those emitted by the active layers by selectively feedingback the light of the specific wavelength, wherein the plurality of theactive layers compensate mutually for variations in the refractive indexinduced by the change of the carrier density.

For realizing the mutual compensation, the plurality of the activelayers are imparted with different gain peak wavelengths. By setting theoscillation wavelength between these peak gain wavelengths, the changeof refractive index brought about by the change in the carrier densitycan be cancelled out through cooperation of the plural active layers,whereby the wavelength chirping can be reduced.

In any of the structures described above, at least one of the pluralactive layers should be disposed in the vicinity of the diffractiongrating (distributed feedback structure) for thereby preventing lossfrom occurring in the grating portion. In this conjunction, with thephrase "disposition in the vicinity of", it is to be understood that theactive layer of concern and the diffraction grating are disposed inparallel and overlapping each other.

Further, according to still another aspect of the invention, there isprovided a semiconductor laser device in which a material capable ofexhibiting the gain peak wavelength shorter than the oscillationwavelength is used for forming the DBR region with a view to suppressingthe gain of the DBR region from being increased by the carrierinjection.

One advantage of the present invention is that notwithstanding of theuse of an active material for forming the DBR region, there can berealized sufficient magnitude of change of the refractive index forprotecting the wavelength-tunable width or range against being narrowedby the self-oscillation, while preventing oscillation threshold in theoptical amplification region from being increased.

Another advantage of the present invention is that an active layerstructure can be adopted in the phase control region for the purpose ofsuppressing the absorption loss which may occur when the passive opticalwaveguide is employed in the phase control region while preventing theoscillation threshold current from being increased. In this manner, losscan be prevented from being increased by the injection of current intothe phase control region, and thus the change in the refractive indexbrought about by the current injection in the phase control region caneffectively maximized, whereby there can be provided a semiconductorlaser device enjoying a broad wavelength-tunable width (range) withoutsuffering any significant fluctuation in the spectral line width and thelight output.

Another advantage of the present invention is that the wavelengthchirping taking place upon direct modulation of the semiconductor lasercan be reduced, whereby optical communication can be realized at a highspeed over an extended distance even when an optical fiber exhibitingnot a less wavelength dispersion is employed.

Another advantage of the present invention is that a significant changein the refractive index can be realized with a small change of the gain(or gain absorption).

Another advantage of the present invention is that the phase and opticalpath length can be changed without being accompanied with variation ofthe light output power, whereby miniaturization of the optical deviceand remarkable improvement in the characteristics of the phase controlregion can be attained.

Still another advantage of the invention is that a signal-modesemiconductor laser which has a broad wavelength-tunable width (range)and oscillation output less susceptible to variation due to change ofthe oscillation wavelength can be realized.

Another advantage of the present invention is that the multiplexednumber and hence the transmission capacity can be significantlyincreased by using the semiconductor laser device according to theinvention as a light source for a wavelength-multiplexed coherentcommunication.

Still further advantages of the present invention will be apparent tothose of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be implemented by various parts and arrangements orcombinations of parts. The drawings are only for the purpose ofillustrating the preferred embodiments of the invention and are not tobe construed as limiting the invention. In the drawings;

FIG. 1 is a vertical sectional view of a semiconductor laser deviceaccording to an exemplary embodiment of the present invention;

FIGS. 2A and 2B are views for illustrating changes of gain and indexrelative to the carrier density in an active optical waveguide;

FIG. 3 is a vertical sectional view showing a structure of awavelength-tunable semiconductor laser device to which the presentinvention is applied;

FIG. 4 is a vertical sectional view of a semiconductor laser deviceimplemented in another structure;

FIGS. 5A to 5F are vertical sectional views for illustrate a man processof semiconductor device according to the present invention;

FIGS. 6 and 7 are sectional views showing semiconductor laser devicesaccording to other embodiments of the invention, respectively,

FIG. 8 is a sectional view showing conventional semiconductor laserdevices;

FIGS. 9A, 9B, 10A, 10B, 11, 12 views showing semiconductor laser devicesaccording to further embodiments of the invention, respectively;

FIGS. 14A and 14B show in a sectional view a semiconductor laser deviceaccording to still another embodiment of the present invention and showin an enlarged sectional view an active layer structure of the same,respectively;

FIG. 15 is a view for illustrating changes of the gain and therefractive index brought about by injection of a in a passive opticalwaveguide;

FIG. 16 is a conceptual view for illustrating changes of the gain andthe refractive index where a case hitherto known optical waveguideexhibiting a high gain is employed in a diffraction grating region;

FIGS. 17A and 17B are views for illustrating optical waveguide asemiconductor laser device according to an exemplary embodiment of theinvention and show a basic structure and energy band at the time of thecarrier respectively;

FIG. 18 is a view for illustrating changes of the gain and therefractive index in an optical waveguide of semiconductor laser deviceaccording to a further embodiment of the present invention; and

FIGS. 19A to 19F views for illustrating a process for a semiconductorlaser device to yet another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, one of the principles underlying the present invention will bedescribed by reference to the accompanying drawings and moreparticularly to FIG. 1 which shows a structure of a wavelength-tunablelaser to which the invention is applied, together with FIGS. 2A and 2Bwhich are views for graphically illustrating changes of gain andrefractive index, respectively, of an active region constituting thepart of the wavelength-tunable laser shown in FIG. 1.

The semiconductor laser device shown in FIG. 1 comprises a distributedBragg (DBR) region 101 including an active optical waveguide 141 havinga first active layer which is so formed on a substrate 106 as to be incontact with a diffraction grating 112, a phase control region 102including an optical waveguide 181 formed of a passive material andhaving a refractive index which decreases as the density of injectedcarriers is increased, and an optical amplification region 103 includingan active optical waveguide 105 having a second active layer structure,wherein each of the three regions 101, 102 and 103 mentioned above isrealized, for example, by a p-i-n junction, being sandwiched between then-type substrate 106 and a p-type cladding layer 107. The semiconductorlaser further comprises independent electrodes 121, 122 and 123 formedon the cladding layer 107 and a common electrode 124 of the polarityopposite to that of the independent electrodes.

In the semiconductor laser, a resonator is formed by reflection from thediffraction grating in the DBR region 101 and reflection from an edge120 of the optical amplification region 103, wherein laser oscillationtakes place upon injection of a current into the optical amplificationregion 103 through the electrode 123 with a large gain. The oscillationwavelength is determined by a wavelength within a Bragg reflectionwavelength range of the DBR region 101 and a wavelength whose phasesatisfies the condition of an integral multiple of 2π upon traveling oneround within the resonator.

Reviewing again the semiconductor laser device disclosed inJP-A-64-49293 cited hereinbefore by reference to FIG. 1, the activeoptical waveguide 105 of the optical amplification region 103 exhibitinga large gain for the injected current has been used as the activewaveguide 141 of the DBR region 101. In conjunction with this prior artstructure, there are illustrated in FIGS. 2A and 2B relations betweenthe carrier density and the gain and the change of the refractive index.The wavelength λ_(Pl) for the gain peak of the active material is soselected as to coincide approximately with the laser oscillationwavlength λ_(L). As a consequence, the gain for the carrier injection atthe laser oscillation wavelength λ_(L) increases rapidly. Thus, aself-oscillation takes place in the DBR region 1. In other words, thesemiconductor laser operates like a distributed feedback (DFB) typelaser. As a result, the carrier density is fixed at a point P shown inFIG. 2A, whereby the change of the refractive index undergoes limitation(see the curve λ_(Pl)).

In contrast, when the active layer which differs from the activewaveguide 105 of the optical amplification region 103 in respect to thechange of gain (differential gain coefficient) for the injected carrierdensity is used as the active optical waveguide 141 of the DBR region101, the change of refractive index can avoid the limitation by theself-oscillation, and besides the loss produced in the passive phasecontrol region can be compensated for. By virtue of these features,there is made available a wide wavelength-tunable width withoutincreasing the threshold current for oscillation of the opticalamplification region. In implementation of such a structure, a materialcapable of emitting the wavelength λ_(P2) at the gain peak which isshorter than the laser oscillation wavelength may be employed for theactive waveguide 141, by way of example. Relations between the carrierdensity and the gain and the change of refractive index for the opticalwaveguide formed of the above-mentioned material are also illustrated inFIGS. 2A and 2B. With the material emitting the shorter wavelengthλ_(P2) at the gain peak, the wavelength λ_(P2) does not coincide withthe laser oscillation wavelength λ_(L), whereby there can be realized agentle gain slope. Consequently, even when the carrier density(injection current) is increased, increasing of photon densityaccompanying the increasing in the carrier density can be suppressed andat the same time the decrease of the carrier density accompanying thesimulated emission which depends on the photon density can besuppressed, to thereby allow the carrier density to be increased. Thus,the gain at which the DBR region 101 is capable of a self-oscillationcan not be reached. Consequently, the carrier density bringing about thechange of refractive index is not fixed but can be made use ofeffectively. Since the change of refractive index brought about by thechange of carrier density is only gently as a function of thewavelength, the change of refractive index brought about by the changeof the carrier density at the laser oscillation wavelength λ_(L) of thematerial emitting the shorter wavelength λ_(P2) at the gain peak canassume the substantially same value as that of the first active layer,although lower than the latter more or less. Thus, it is possible toincrease the wavelength-tunable width and in particular the Braggreflection wavelength-tunable width.

As will be appreciated from the above description, it is possible toobtain broader wavelength-tunable characteristics while suppressing theoscillation threshold current from being increased in the opticalamplification region 101 by making smaller the gain of the DBR regionfor a carrier density than that of the optical amplification region.

For the phase control, a current is injected in the phase control region102 to thereby match or align the phase with the axial-mode resonancecondition for the laser light between the Bragg reflection region andthe cleavage edge 120, whereby the oscillation wavelength can becontinuously shifted over a wide wavelength range in the single-modestate. Further, since the change of gain as a function of the change inthe injected carrier density is flattened, there can be obtained anadvantageous effect that the change in the emission power occurring uponshifting the oscillation wavelength can be noticeably suppressed.

Further, by employing such a material for the active optical wavelength105 constituting the optical amplification region 103 which can have alonger gain peak wavelength than the oscillation wavelength, theoscillation wavelength can be detuned to be shorter than the gain peakwavelength. By virtue of such detuning, the ratio between the change ofthe refractive index which accompanies fluctuation of the carrierdensity and change of the gain, i.e. a so-called α-parameter can bereduced, as a result of which the spectral line width is diminished, toan advantageous effect.

Now, another one of the principles underlying the present invention willbe described by reference to FIG. 3 in which same reference symbols areused for designating like constituents as those shown in FIG. 1, (beingunderstood that the same applies true throughout several figuresreferred to in the description which follows). In the semiconductorlaser structure shown in FIG. 3, the structure of the DBR region and theoptical waveguide structure of the phase control region shown in FIG. 1are mutually replaced such that a passive optical waveguide 382 having arefractive index which is decreased upon injection of carriers is usedin the DBR regions with a second active optical waveguide 342 being usedin the phase control region 102. With this structure, increasing of theabsorption loss observed heretofore in the passive phase control regionas a result of increasing in the carriers can positively be prevented.To this end, however, it is required to make greater the ratio betweenthe change of refractive index of the phase control region 102 and thechange of gain (i.e. refractive index change/gain change ratio) thanthat of the optical amplification region 103. Besides, due to thecombination with the passive DBR region, the loss taking place in theDBR region can be canceled out by a gain obtained in the phase controlregion 102, whereby the oscillation threshold of the opticalamplification region 103 is prevented from increasing.

Further, when an optical waveguide 433 having a rate of change of therefractive index substantially equal to that of the optical waveguide141 shown in FIG. 1 is employed for the phase control region 102 inaddition to that for the DBR region 101, the threshold current isprevented from increasing, whereby the self-oscillation of the DBRregion 101 can be eliminated as well. This characteristic can berealized by selecting the material 433 such that the gain peakwavelength λ_(P2) becomes shorter.

As described above, the active optical waveguide exhibiting the gainpeak wavelength lying on a short wavelength side relative to the laseroscillation wavlength is employed for the DBR region and/or for thephase control region. This structure can be realized by making thecomposition of the waveguide differ from that of the amplificationregion. As an alternative, the thickness of the active waveguide may bemade smaller than that of the amplification region. In the latter case,the gain peak wavelength is shifted toward the shorter wavelength side,as the carrier density is increased, whereby the peak wavelength can bemade shorter than the laser oscillation wavelength. Besides, byimplementing the active optical waveguide in a multi-layer structureincluding an optical guide layer and an active layer having a thicknesssmaller than that of the former, a part of the injected carriers leaksinto the optical guide layer, as a result of which the self-oscillationof the DBR region is suppressed while allowing the refractive index tochange noticeably.

Thus, according to the teaching of the invention described above, theself-oscillation of the distributed Bragg reflection (DBR) region can besuppressed regardless of the current injection into the active layereither by realizing the DBR region in the structure differing from thatof the optical amplification region and employing for the DBR region theactive layer which can assure a significant change in the refractiveindex substantially to the same extent as the active layer exhibitingthe gain peak close to the oscillation wavelength notwithstanding of asmall gain or alternatively by employing for the phase control regionthe active layer structure which exhibits a smaller gain for theinjected current than the optical amplification region and which canensure more significant change in the refractive index.

The other principle of the invention will be described by reference toFIG. 11. In order to reduce the chirping in wavelength, it is requiredthat a plurality of regions provided in the semiconductor laserresonator (cavity) cooperate mutually to compensate for fluctuation ofchirping in the wavelength. FIG. 11 shows an example of a structurewhich can satisfy the above requirement. Referring to the figure, thereare provided in the vicinity of a diffraction grating 1112 (on anoptical guide layer 1113) active layers 1104 and 1105 having mutuallydifferent compositions so that the regions 1104 and 1105 differ fromeach other in respect to the gain peak wavelength. A pair of discreteelectrodes 1108 and 1109 are provided on a cap layer 1118 for injectingcarriers into the regions 1104 and 1105 separately. The different gainpeak wavelengths result in difference in the carrier density dependenceof the gain (differential gain) at the laser oscillation wavelength.Upon occurrence of the laser oscillation in this semiconductor laserdevice in response to application of current pulse, the carrier densitychanges at different rates in the individual regions. Thus, although thecarrier density is certainly decreased by the stimulated emissionbrought about by the laser oscillation, the laser light intensity isprevented from increasing excessively owing to a remarkable change ofthe gain in the region having a large differential gain, while the laserlight intensity is protected against excessive attenuation due to agentle change of the gain in the region having a small differentialgain. In this manner, the rate of transient change in the carrierdensity can be made low. Although the refractive index depends on thecarrier density, it is an average refractive index of the whole devicethat determines the laser oscillation wavelength. Thus, according to theprinciple described above, the magnitude of fluctuation in theoscillation wavelength, i.e. the wavelength chirping can be diminishedwith the laser structure shown in FIG. 11 by virtue of smaller change inthe carrier density on an average.

In order to set the gain peak wavelengths such that they assumespatially different values, there may be conceived several methods, someof which will be mentioned below. First, by varying the composition ofthe active layers in dependence on the positions thereof within theresonator or by varying the thickness from one to another active layers,the gain peak wavelengths are changed by taking advantage of the bandfilling effect of the carriers. Further, the regions differing from oneanother in the gain peak wavelength may be disposed in cascade in thedirection of the optical axis or in parallel so far as they are locatedwithin the range of light intensity profile. The laser oscillationwavelength should preferably be set intermediate between the differentgain peak wavelengths, because then the difference in the differentialgain can be used very effectively.

As will be appreciated from the foregoing description, the regionsmutually differing in the gain peak wavelength in the semiconductorlaser device also differ from one another in the differential gain forthe laser oscillation wavelength. By virtue of this difference in thedifferential gain, variation in the refractive index is suppressed.

On the principle, each of the regions may assume the gain state or lossstate at the laser oscillation wavelength. However, excessively highloss in the loss state involves increasing in the threshold current aswell as degradation in the high-speed characteristics. Accordingly, theregion should preferably be set to the state at least close to the gainstate.

In order to make the most of the difference in the differential gain,the period of the diffraction grating should desirably be dimensionedsuch that the wavelength determined thereby lies intermediate betweenthe gain peak wavelengths of the individual regions.

In connection with magnitude of difference in the gain peak wavelength,it should be mentioned that too small difference can not bring about theaimed effect, while excessively large difference involves a noticeableloss in one of the regions at the laser oscillation wavelength,involving an increase in the threshold current. Under the circumstances,the wavelength difference mentioned above should preferably be selectedin a range of 5 nm to 100 nm and more preferably in a range of 10 nm to50 nm.

It is thus apparent that the chirping in the wavelength can be reducedby providing within the resonator a plurality of regions differing fromone another in respect to the gain peak wavelength or the gain slope forthe injected carrier density.

Now, the present invention will be described in greater detail inconjunction with preferred or exemplary embodiments thereof.

Embodiment 1

FIG. 5F shows a structure of a semiconductor laser device according to afirst exemplary embodiment of the invention in a sectional view takenalong the optical axis. A process for manufacturing this semiconductorlaser device will be described below by reference to FIGS. 5A to 5F. Atfirst, a diffraction grating 512 (having a pitch of 240 nm) is formedpartially or locally on an n-type InP-substrate 506, whereon there areformed sequentially through crystal growth process an n-typeInGaAsP-guide layer 513 (exhibiting a gain peak wavelength λ_(P) of 1.3μm), an n-type InP-stopping layer 514, an i-type InGaAsP-active layer519 exhibiting a gain peak wavelength λ_(P) shorter than the oscillationwavelength (i.e. λ_(P) in a range of 1.50 to 1.53 μm), and an i-typeInGaAsP-antimeltback layer 516 (exhibiting a gain peak wavelength λ_(P)of 1.3 μm), as shown in FIG. 5A, wherein the values of the gain peakwavelengths λ_(p) mentioned above are measured in the vicinity of theoscillation threshold currents in the respective materials. Next, aportion having the grating 512 is masked with a photoresist 525, whereonthe antimeltback layer 516, the active layer 519 and the stopping layer514 in the other portion are selectively etched by etchants appropriateto these layers, respectively, with the guide layer 513 being left as itis (FIG. 5B). After removing the photo-resist layer 525, theInP-stopping layer 514', the InGaAsP-active layer 515 having the gainpeak wavelength in the vicinity of the oscillation wavelength (i.e.λ_(P) =1.55 μm) and the InGaAsP-antimeltback layer 516' (λ_(P) =1.3 μm)are sequentially grown through crystal growth process (FIG. 5C). Next, aportion destined to constitute an optical amplification region isprotected by a photoresist 525', whereon the antimeltback layer 516',the active layer 515 and the stopping layer 514' in the other portionare selectively etched by etchants appropriate to these layers,respectively (FIG. 5D). After removing the photoresist 525', a p-typecladding layer 517, and a p⁺ -type InGaAsP-cap layer 518 (λ_(P) =1.15μm) are sequentially grown (FIG. 5E), being then followed by formationof waveguide stripe, burying crystal growth and others. Finally, p-typediscrete electrodes 521, 522 and 523 are formed on the surfaces of a DBRregion 501, a phase control region 502 and an optical amplificationregion 503, respectively, while an n-type electrode is formed over thelower surface of the substrate 506, as shown in FIG. 5F.

It should be mentioned that the semiconductor laser device of astructure similar to that described above may also be manufactured byusing insulation films such as of SiO₂ or the like in the crystal growthprocess. More specifically, there are formed on the substrate 506 havingthe diffraction grating 512 deposited thereon the guide layer 513, thestopping layer 514, the active layer 519 and the antimeltback layer 516of the DBR region 501 mentioned above through epitaxial growth process.Thereafter, the active DBR region 501 is protected by a SiO₂ -mask,whereon the phase control region 502 and the optical amplificationregion 503 are selectively etched away until the InP-stopping layer 514is reached. Subsequently, the multi-layer film for the active opticalwaveguide in the optical amplification region 503, i.e. the multi-layerfilm including the i-type InGaAsP-active layer 515 (λ_(P) =1.55 [m) andthe i-type InGaAsP-antimeltback layer 516'(λ_(P) =1.3 μm) is formedthrough epitaxial growth. Since the active DBR region 1 is protected bythe SiO₂ -mask at that time, only the phase control region 502 and theoptical amplification region 503 are allowed to selectively growepitaxially. subsequently, the active DBR region 501 and the opticalamplification region 503 are protected by the SiO₂ -mask to etch awayselectively the phase control region 502 until the InP-stopping layer514 is reached. After removing the SiO₂ -mask, the p-type InP-layer 517and the P⁺ -type cap layer 518 are grown epitaxially, which is thenfollowed by mesa-etching and burying epitaxial growth to thereby form aburied hetero-structure. Finally, a p-type electrode layer is depositedby evaporation and then separated into the electrode 521 for the activeDBR region, the electrode 522 for the phase control region and theelectrode 523 for the optical amplification region 523. Finally, then-type electrode 524 is formed over the lower surface of the substrate.

It should be mentioned that the first mentioned method is advantageousover the second mentioned method in that the surface resulting from thecrystal growth is more smooth because there is no need for covering theregions not to be grown by the insulation film in the epitaxial growthprocess.

Next, description is directed to operation of the semiconductor laserdevice according to the instant embodiment.

The condition for the laser oscillation resides in that the gaininternally of the laser is so balanced that the intensity of lightmaking a round trip within the laser coincides with the originalintensity and that the phase of light making the one round trip is givenby an integral multiple of 2π. In the case of the semiconductor laserdevice of the structure described above, when the gain of light due tothe current I_(a) injected into the optical amplification region 503,the attenuation due to absorption by the free carriers ascribable to theinjected current I_(P) in the phase control region 502, the gain due tothe injected current I_(b) in the DBR region and the loss in thequantity of light upon leaving the laser are balanced with an overallgain of unity (one), then oscillation takes place at a wavelength forwhich a sum of the phase of reflection in the active DBR region 501 asviewed from the phase control region 502, the phase of propagation inthe phase control region 502 and the optical amplification region 503and the phase of reflection at the edge 520 of the optical amplificationregion 502 is given by an integral multiple of 2π. When only theinjection current to the active DBR region 501 is increased in the statein which the laser is oscillated by injecting a constant current to theoptical amplification region 503, the Bragg wavelength is shifted about-0.04 nm/mA to the shorter wavelength side. Further, when only thecurrent injected to the phase control region 502 is increased, the Braggwavelength is also shifted about -0.1 nm/mA to the shorter wavelengthside.

In this manner, when the current is injected into the phase controlregion 502 simultaneously with the current injection into the active DBRregion 501 with a proper ratio to the injection current I_(b) for theDBR region 501 (e.g. with a ratio of about 1/2 for the values mentionedabove) while maintaining constant the injection current I_(a) to theoptical amplification region 503, an optical path length control is soperformed as to satisfy the oscillation phase condition for the shift ofthe Bragg wavelength.

Thus, it is possible to shift continuously the oscillation wavelengththrough current injections to the active DBR region 501 and the phasecontrol region 502. Since the material employed for the active layer 519constituting the active DBR region 501 exhibits the gain peak wavelengthshorter than the oscillation wavelength in the case of the semiconductorlaser according to the instant embodiment, increasing of the gain due tothe current injection can be flattened, whereby a higher carrier densitycan be obtained with increase of the photon density being suppressedeven in the region injected with a large current, thereby making itpossible to shift the Bragg wavelength. Besides, because of littleincrease in the photon density, the oscillation power can be protectedagainst any noticeable variation even when the oscillation wavelength ischanged.

The characteristics of the wavelength-tunable semiconductor laser deviceof the structure described above is evaluated as follows. By controllingthe current injected into the DBR region 501 and the phase controlregion 502 while supplying a constant current in a range of 100 mA tothe optical amplification region 503, a continuously wavelength tunablewidth of 5 nm can be realized with the laser power of 10 mW emitted fromthe edge 520 of the optical amplification region 503. Even when thecurrent injection to the DBR region 501 is increased, there occurs noclamping of the wavelength shift due to a self-oscillation of the DBRregion 501. In the oscillation spectra, there exists no spectrumoriginating in the self-oscillation. Only the single spectrum formed bythe edge 520 of the optical amplification region 503 and the DBR region501 can be observed.

Embodiment 2

As a dominant factor for increasing the oscillation threshold, there canbe mentioned an increase in the absorption loss which accompanies theincrease in the current injected to the phase control region formed of apassive material. The second embodiment of the invention is directed toa structure of the semiconductor laser in which a material exhibitingthe gain peak at a shorter wavelength than the oscillation wavelength isemployed for the phase control region. FIG. 6 shows a structure of thesemiconductor laser device according to the second embodiment of theinvention in a section taken along the optical axis. The manufacturingprocess of this laser device is substantially same as that for the laseraccording to the first embodiment described hereinbefore by reference toFIGS. 5A to 5F except for a difference in that at the step shown in FIG.5B, the phase control region located intermediately is left in place ofthe multi-layer DBR region stacked first. Subsequently, through thesucceeding steps similar to those in the first embodiment, there isrealized the structure shown in FIG. 6.

The characteristics of the wavelength-tunable semiconductor laseraccording to the instant embodiment have been evaluated. By controllingthe currents injected into the DBR region 501 and the phase controlregion 502 while maintaining constant at 100 mA the injection current tothe optical amplification region 503, a continuously wavelength tunablewidth of 4.5 nm can be realzied characteristically. The laser powerundergoes substantially no variation and can be maintained around 7 mW.This in turn means that the oscillation threshold scarcely changes.

Embodiment 3

Starting from the semiconductor laser structure according to the secondembodiment, it is taught by the invention incarnated in the thirdembodiment to provide gain regions in the DBR region and the phasecontrol region, respectively, for the purpose of suppressing occurrenceof the absorption loss ascribable to the use of the passive material.FIG. 7 shows a structure of the wavelength-tunable semiconductor laseraccording to the third embodiment in a section taken along the opticalaxes. With the structure according to the instant embodiment, it isnecessary to shift the gain peak wavelength to the shorter wavelengthside relative to the laser wavelength, because there is no necessity formaking up the absorption loss in the passive material, as in the case ofthe first and second embodiments. It is now assumed that the gain peakwavelength of the active layer 526 for the DBR region 501 and the phasecontrol region 502 is 1.47 μm in the structure shown in FIG. 7. Theprocess for manufacturing the semiconductor laser device according tothe instant embodiment is substantially same as that in the firstembodiment described hereinbefore by reference to FIGS. 5A to 5F exceptfor difference in that at the step for forming first and multi-layerfilm (FIG. 5A), the active layer 519 exhibiting the peak gain wavelengthλ_(P) of 1.50 μ m is replaced by an InGaAsP-active layer 525 (λ_(P)=1.47 μm) and that at the etching process shown in FIG. 5B, both the DBRregion 501 and the phase control region 502 are covered with aphotoresist for removing the optical amplification region 502 to the topof the guide layer by etching. Subsequent process steps are carried outin the same manner as in the case of the first embodiment.

According to the characteristics of the wavelength-tunable semiconductorlaser realized in the structure described above, there can be realized acontinuous wavelength-tunable width or range of 5 nm by controlling thecurrents injected into the DBR region 501 and the phase control region502 while maintaining constant at 100 mA the current injected to theoptical amplification region.

Embodiment 4

The fourth embodiment of the invention is concerned with a structure ofthe wavelength-tunable semiconductor laser device in which an activelayer capable of giving birth to a gain of such magnitude as to makingup the absorption loss occurring upon injection of carriers in thepassive guide layer is joined to the passive guide layer in the DBRregion or the phase control region in parallel to the direction in whichthe light is guided. FIG. 17A is a sectional view showing a basicstructure of the optical waveguide of the semiconductor laser deviceaccording to the fourth embodiment of the invention, and FIG. 17B is aview for illustrating an energy band at the time of carrier injection inthe device according to the instant embodiment. Disposed as sandwichedbetween a p-type cladding layer 1745 and an n-type cladding layer 1741are guide layers 1742 and 1744 of lower bandgap energy than the claddinglayers, wherein an active layer 1743 of lowest bandgap energy isdisposed as sandwiched between the guide layers 1742 and 1744. In thisconjunction, it should however be noted that the provision of only oneguide layer may also be sufficient. Basically, the semiconductor layeraccording to the instant embodiment is similar to a double hetero typesemiconductor laser. A characteristic feature of the semiconductor laseraccording to this embodiment is seen in that the thickness of the activelayer 1743 is diminished while increasing the thickness of the guidelayers 1742 and 1744. In an ordinary semiconductor laser havingoscillation wavelength of 1.55 μm, the active layer is imparted with athickness not smaller than 0.1 μm in order to obtain oscillation gain.Consequently, in the case of the ordinary semiconductor laser having thethick active layer 1743 as mentioned above, the current injected istransformed to carrier electrons 1746 and carrier holes 1747. On theother hand, substantially all of the carriers injected remain within theaforementioned active layer 1743 and scarcely overflow to the guidelayers 1742 and 1744. The thickness of the active layer which exceeds1/15 of the wavelength, as described above, leads to the laseroscillation. Under the circumstance, the thickness of the active layeris selected smaller than 1/15 of the wavelength intended for use, whilethat of the passive guide layer is selected to be larger than 1/15 ofthe wavelength. In FIG. 17B, a reference symbol E_(c) representsconduction band edge energy, E_(v) represents valence band edge energy,φ_(v) represents hole energy and φ_(c) represents electron energy.Further, hatched areas indicate regions in which electrons are present.

By dimensioning the thickness of the active layer not greater than 0.07μm (=1/15 of the wavelength for use) with the thickness of the guidelayer being not smaller than 0.15 μm, that portion of the waveguidewhich is occupied by the active layer 1743 is reduced, whereby the gainof the waveguide is decreased to an extent effective for suppressing theself-oscillation. Further, because of thickness of the active layer1743, the injected carriers can overflow to the guide layers 1742 and1744. As a result of this, the refractive indexes of the guide layers1742 and 1744 are forced to change, and at the same time the carriersare absorbed by the guide layers 1742 and 1744 mentioned above, which inturn is effective for further suppressing the self-oscillation. In thisway, by placing the thin active layer in contact with the passive guidelayers 1742 and 1744, the self-oscillation can be suppressed with lesschange of the gain for the injection of carriers. On the other hand, theabsorption of carriers by the guide layers can not lead to the cease ofoscillation. Consequently, the significant change in the refractiveindexes brought about by the injection of carriers into the active layer1743 and the guide layers 1742 and 1744 can be made use of to a possiblemaximum. In this manner, there can be realized a greater wavelengthtunable width or range without deteriorating the characteristics of thewavelength-tunable laser.

When such a material is employed for the active layer which has ashorter gain peak wavelength than the oscillation wavelength, the gaincan be suppressed to 30 cm⁻¹ or less. Thus, in this case, theself-oscillation can be suppressed while allowing greater refractiveindex to be obtained, even when the active layer has a thickness greaterthan 0.07 μm inclusive.

Now, description will be made in detail of the semiconductor laseraccording to the instant embodiment by referring to FIGS. 9A and 9B, inwhich FIG. 9A shows a vertical sectional view of the same taken alongthe line A--A' in FIG. 9B which shows a cross-sectional view. Adiffraction grating 912 is formed locally on an n-type InP-substrate906. Subsequently, there are formed sequentially an InGaAsP-guide layer914 exhibiting a bandgap wavelength λ_(P) of 1.3 μm in a thickness of0.2 μm, an n-type InP-stopping layer 907 in a thickness of 0.03 μm, anInGaAsP-active layer 913 having λ_(P) of 1.55 μm in a thickness of 0.10μm, and an InGaAs-antimeltback layer 904 having λ_(P) of 1.3 μm in athickness of 0.04 μm through crystal growth process. Thereafter, theregions 901 and 902 exclusive of the optical amplification region 903are etched till the stopping layer 907. Then, there are formed over thewhole surface an InGaAsP-active layer 905 having λ_(P) of 1.55 μm in athickness of 0.05 μm, an antimeltback layer 908 in a thickness of 0.04μm, a p-type InP-cladding layer 920 in a thickness of 2 μm, and anInGaAsP-cap layer 918 having λ_(P) of 1.15 μm in a thickness of 0.5 μmthrough crystal growth process. Next, mesa reversal etching is performedto an extent that the active layer remain in a width of about 1 μm (FIG.9B). Thereafter, a p-type InP-layer 910, an n-type InP-burying layer911, p-type electrodes 921, 922 and 923 and an n-type electrode 924 areformed. The p-type electrodes are provided separately for thediffraction grating region 901, the phase control region 902 and theoptical amplification region 903, respectively.

With the structure of the semiconductor laser described above,increasing of the current injected into the diffraction grating region901 does not result in occurrence of the self-oscillation. Thecharacteristic evaluation shows that the gain is not greater than 30cm⁻¹. A continuously tunable wavelength width of 4 nm can be obtainedfor the oscillation wavelength.

Embodiment 5

The semiconductor laser according to this embodiment has a structurediffering from that shown in FIG. 9 in respect that an InGaAsP-layer(λ_(P) =1.52 μm) having a thickness of 0.06 μm is employed for theactive layer of the diffraction grating region 901 and the phase controlregion 902. According to embodiment, the continuously tunable wavelengthwidth is increased to 5 nm.

Embodiment 6

In the structure shown in FIG. 9, the active layer similar to thatdescribed hereinbefore in conjunction with the first embodiment isemployed in a thickness of 0.06 μm with the guide layer being formed ofan InGaAsP-layer (λ_(P) =1.38 μ). According to the sixth embodiment, acontinuously tunable wavelength width of 5 nm can be realized.

Embodiment 7

A seventh exemplary embodiment of the invention will be described byreference to FIG. 10. There are formed on a n-type InP-substrate 1006through crystal growth process InGaAs-layers 1004, 1005 and 1007 havingλ_(P) of 1.3 μm, 1.55 μm and 1.3 μm, respectively, and in thicknesses of0.10 μm, 0.15 μm and 0.05 μm, respectively. Thereafter, the regions 1001and 1002 are removed by etching with optical amplification region 1003being left as it is, whereon layers 1008, 1009 and 1010 having λ_(P) of1.3 μm, 1.55 μm and 1.3 μm are grown in thickness of 0.2 μm, 0.05 μm and0.1 μm, respectively, through vapor phase growth. Subsequently, throughthe process steps corresponding to those described hereinbefore inconjunction with the fourth embodiment, the semiconductor laser of astructure shown in FIG. 10B is realized.

With the laser structure according to the instant embodiment, not onlythe continuously tunable wavelength width substantially equal to that ofthe first embodiment can be obtained but also a lower threshold current(i.e. current injected to the optical amplification region) can beachieved.

Embodiment 8

A three-electrode wavelength-tunable laser manufacturing method whichdiffers from that described hereinbefore in conjunction with the firstembodiment will be described by referring to FIGS. 19A to 19F.

At first, there are formed on an n-type InP-substrate 1906 inscribedlocally with a diffraction grating 1912 an n-type InGaAs-guide layer1913 of λ_(P) equal to 1.3 μm, an InGaAsP-active layer 1919 of λ_(P)equal to 1.53 μm and an anti-meltback layer 1916 of λ_(P) equal to 1.3μm through epitaxial growth process (FIG. 19A). While protecting by aphotoresist mask 1925 the region where the diffraction grating 1912 isprovided, the other grown region are etched by using a selective liquidetchant containing H_(2l) SO₄, H₂ O₂ and H₂ O (FIG. 19B).

Next, after having removed the photoresist mask 1925, an InGaAsP-layer(1916/1913) of λ_(P) =1.3 serving an an anti-meltback layer and a guidelayer, an InP-stopping layer 1914, an active layer 1915 of λ_(P) equalto 1.55 μm and an anti-meltback layer 1916' are formed over the wholesurface through epitaxial growth. Subsequently, a flat surface portionof the substrate surface 1906 is protected by a photoresist mask 1925',and then the anti-meltback layer 1916' and the active layer 1915 in theother regions are selectively etched by using the liquid etchantmentioned above. Further, only the InP-stopping layer 1914 isselectively etched by using a liquid etchant of H₂ PO₄ /HCl-series.

After the photoresist mask 1925' is removed, a p-type InP-cladding layer1917 and a cap layer 1918 are formed through epitaxial growth (FIG.19E). Finally, Zn is diffused into electrode junctions to thereby form ap-type electrode 1921 in the DBR region 1901, a p-type electrode 1922 inthe phase control region and a p-type electrode 1923 in the opticalamplification region 1903 while an n-type electrode 1924 is formed overthe rear surface of the substrate (FIG. 19).

According to the manufacturing method described above, no measures aretaken for covering the region not to be grown in the epitaxial growthprocess. By virtue of this feature, the surface resulting from thecrystal growth becomes smooth with an enhanced optical coupling beingrealized among the individual regions. Besides, the manufacturingprocess can be carried out with improved accuracy. Thus, a semiconductorlaser of a lower threshold current than that of the laser according tothe first embodiment can be manufactured more easily.

When the manufacturing method described above is adopted, the opticalcoupling can be realized more smoothly not only between the twodifferent active optical waveguides but also among three or moredifferent active waveguides.

In the semiconductor laser according to the instant embodiment, therearises a possibility that the film thickness of the anti-meltback layer1916 of the active DBR-region 1901 may become excessively thick, asresult of which the light intensity distribution can spread in thisregion. Accordingly, the structure shown in FIG. 5F is advantageous overthe instant embodiment in that the enhanced inter-region opticalcoupling is realized to facilitate the manufacturing of a low thresholdsemiconductor laser. However, even in the semicondcutor laser accordingto the instant embodiment, similar advantage can be assured by selectingappropriately the refractive index of the material for the anti-meltbacklayer 1916.

Embodiment 9

The semiconductor laser according to the ninth embodiment starts fromthe structure shown in FIG. 5F and differs from the latter in that thecomposition of the InGaAsP-active layer 515 constituting the opticalamplification region 503 is so selected that the gain peak wavelengthemitted thereby is about 1.56 μm longer than the oscillation wavelengthof 1.55 μm. Due to this structure, the detuning effect is obtained tothereby allow the spectral line width to be more narrower than that ofthe device according to the first embodiment.

In the case of the first of eighth embodiments described above, the gainpeak wavelength emitted by the material used for the active opticalwaveguide in the DBR region is shorter than the oscillation wavelengthby 0.02 μm. In this conjunction, it is to be mentioned that similareffects can be achieved so long as such differs in the wavelength lieswithin a range up to 0.06 μm. In particular, in the range from 0.01 μmto 0.06 μm, there can be attained an advantageous effect that theoscillation power is increased owing to the reflection gain in additionto the effects of the broad oscillation wavelength-tunable width andscarcely appreciable variation in the laser powder regardless of changein the oscillation wavelength.

In the foregoing, several examplary embodiments of the present inventionhave been described in conjunction with the wavelength-tunabledistributed Bragg reflection (DBR) type semiconductor lasers. It shouldhowever be understood that the present invention my equally be appliedto other optical devices such as Mach-Zender interferometers, compositecavity type semiconductor lasers and the like in which parts arerequired for changing the optical path length while maintaining constantthe gain (loss) in the propagation of light.

Embodiment 10

Now, referring to FIG. 11, description will be directed to a tenthexemplary embodiment of the invention which is applied to a high-speedmodulating semiconductor laser. An n-type InP-substrate 106 (having athickness of 100 μm) is formed with a diffraction grating 1112 (having aperiod of 0.24 μm) by an interference exposure method, whereon there aregrown an n-type InGaAsP-optical guide layer 1113 (exhibiting a bandgapwavelength λ_(P) =1.3 μm and having a thickness of 0.15 μm), and a firstundoped InGaAsP-active layer 1104 (having λ_(P) equal to 1.56 μm and athickness of 0.15 μm). Subsequently, the active layer 1104 is removedfor an area corresponding to a half of the device by a chemical etching,being followed by growing a second undoped InGaAsP-active layer 1105(λ_(P) =1.53 μm, a thickness =0.15 μm) over the area where the activelayer 1104 has been removed. Thereafter, a p-type InP-cladding layer 120(having a thickness of 2 μm) and a p-type InGaAsP-cap layer 1118 (λ_(P)=1.15 μm, a thickness =0.5 μm) are grown. Finally, Cr-Au electrodes 1108and 1109 and an AuGeNi-Au electrode 124 are formed through evaporation.In this device, the period of the diffraction grating is so selectedthat the laser oscillation wavelength is 1.55 μm which lies intermediatebetween the gain peak wavelengths of the active layers 1104 and 1105.Thus, difference in the differential gain between them can be usedeffectively .

When the laser output light is modulated by applying a pulse signal of2.4 Gb/s to the electrode 1108 in the state in which DC currents areapplied to the electrodes 1108 and 1109, respectively, the totalspectral width has a spread of 1 Å (angstrom), which is apparentlydecreased to 1/5 of that of the prior art modulating laser device.Similarly, when the pulse signal is applied to the electrode 1109 insuperposition to the DC current, the spectral width is decreased toabout 1/4 of that of the prior art device.

Embodiment 11

Another embodiment of the invention is shown in FIG. 12. The instant(eleventh) embodiment differs from the tenth embodiment shown in FIG. 11in that the second undoped InGaAsP-active layer 1105 is replaced by anundoped InGaAsP-active layer 1205 (λ_(P) =1.56 μm, thickness =0.06 μm)and a p-type InGaAsP-optical guide layer 1204 (λ_(P) =1.3 μm, thickness=0.09 μm) grown sequentially.

By applying DC currents of substantially same magnitude to theelectrodes 1109 and 1108, the carrier density is increased because ofthe thickness of the active layer 1205, resulting in that the gain peakwavelength is caused to shift by 1.53 μm in the direction toward theshorter wavelength side due to the band filling effect. This embodimentis also effective to reduce the wavelength chirping as in the case ofthe tenth embodiment.

Embodiment 12

A further (twelfth) embodiment of the invention will be described byreference to FIG. 13. The instant embodiment differs from the tenth andthe eleventh in that a quantum well structure is adopted in the activelayer. In other words, the active layer is realized in multiple quantumwells. Referring to FIG. 13, there is formed on a right-hand side region1305 a stack of undoped InGaAsP-barrier layers (each having a bandgapwavelength of 1.35 μm and a thickness=100 Å) and undoped InGaAs-welllayers (each having a bandgap wavelength of 1.65 μm and a thickness =60Å) stacked alternately in ten cycles (pairs). On the other hand, thereis formed on the left-hand side region 1305' a stack of undopedInGaAsP-barrier layers (each having a bandgap wavelength of 1.3 μm and athickness =100 Å) and undoped InGaAs-well layers (each having a bandgapwavelength of 1.65 μm and a thickness of 60 Å) stacked alternately inten cycles.

The gain peak wavelength of the region 1305 is 1.56 μm, while that ofthe region 1305' is 1.53 μm. Thus, the wavelength chirping can bereduced as in the case of the other embodiments.

Embodiment 13

Still another embodiment of the invention is shown in FIGS. 14A and 14B.Referring to the figures, grown sequentially on an n-type InP-substrate1406 having diffraction grating formed thereon are an n-typeInGaAsP-optical guide layer 1413 (having λ_(P) of 1.3 μm and a thicknessof 0.15 μm), a multiple quantum well active layer 1405 and a p-typeInP-cladding layer 1420 (of 2 μm in thickness) and a p-type InGaAsP-caplayer 1418 (having λ_(P) of 1.15 μm and a thickness of 0.5 μm), whereona Cr-Au electrode 1408 and an AuGeNi-Au electrode 1424 are formedthrough evaporation. The active layer 1405 is implemented in a stackedstructure in which an InGaAsP-barrier layer 1405b (having λ_(P) of 1.3μm and a thickness of 50 Å), an undoped InGaAs-well layer 1405a (of 60 Åin thickness) and an undoped InGaAs-well layer 1405c (of 70 Å inthickness ) are stacked in such a manner as shown in FIG. 14B.

The gain peak wavelength of the well layer 1405a is 1.54 μm because ofthinness thereof, while that of the well layer 1405c is 1.5 μm, whereinthe oscillation wavelength of the diffraction grating 1412 is 1.55 μm.Thus, the wavelength chirping can be reduced as in the case of the otherembodiments.

The foregoing description has been made on the assumption that materialsof InGaAsP-series are used. However, it goes without saying that the useof other materials such as of InGaAlAs-series, GaAlAs-series and thelike can assure similar advantageous action and effects.

Although the invention has been described with reference to thesemiconductor laser devices shown in the drawing, it is to beappreciated that the invention are applicable to other optical devices,including interferometers, resonators, and the like.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alternations will occur tothose of ordinary skill in the art upon reading and understanding thepresent specification. It is intended that the invention be construed asincluding all such alterations and modifications in so far as they comewithin the scope of the appended claims or the equivalent thereof.

What is claimed is:
 1. A semiconductor laser device, comprising:aplurality of semiconductor regions having active layers opticallycoupled to one another and susceptible to undergo changes in gain uponinjection of carriers; said plurality of semiconductor regions includingan amplification region having an optical amplification active layer foremitting light in response to the injection of the carriers, and a DBRregions having a gain active layer for guiding the light emitted by saidoptical amplification active layer and a distributed feedback structurefor feeding back the light being guided; means for injecting thecarriers into said plurality of semiconductor regions; and a resonatorstructure for amplifying the light of a specific wavelength of thoseemitted by said optical amplification active region by selectivefeedback through said feedback structure; wherein differential gaincoefficient to the injected carrier density of said gain active layer ismade different from the differential gain coefficient to the injectedcarrier density in said optical amplification active layer.
 2. Asemiconductor laser device according to claim 1, wherein a semiconductormaterial constituting said optical amplification layer is different froma semiconductor material constituting said gain active layer.
 3. Asemiconductor laser device according to claim 1, wherein said pluralityof active layers have a quantum well structure.
 4. A semiconductor laserdevice according to claim 1, wherein the differential gain coefficientof said gain active layer is smaller than that of said opticalamplification active layer.
 5. A semiconductor laser device according toclaim 1 further comprising a phase control region including an opticalwaveguide layer for light propagation formed of an active semiconductormaterial.
 6. A semiconductor laser device according to claim 5, whereinsaid phase control region includes an electrode for changing an opticalpath length of said resonator.
 7. A semiconductor laser device accordingto claim 5, wherein said optical amplification active layer and saidgain active layer are optically coupled directly or indirectly throughsaid phase control region.
 8. A semiconductor laser device according toclaim 1 further comprising a phase control region including an opticalwaveguide layer for light propagation formed of a passive semiconductormaterial.
 9. A semiconductor laser device according to claim 7, whereinsaid optical waveguide layer is formed of an active semiconductormaterial.
 10. A semiconductor laser device according to claim 1, whereinsaid DBR region includes an electrode for changing the refractive indexof said distributed feedback structure.
 11. A semiconductor laserdevice, comprising:a plurality of semiconductor regions including aplurality of active layers coupled optically and having gainssusceptible to change upon injection of carriers; means for injectingthe carriers into said plurality of the active regions; and a resonatorstructure for amplifying and oscillating light of a specific wavelengthdiffering from said gain peak wavelengths by selectively feeding backthe light of said specific wavelength; wherein said plurality of theactive layers compensate mutually for variations in the refractive indexinduced by variations in the carrier density.
 12. A semiconductor laserdevice according to claim 11, wherein said plurality of the activelayers exhibit different gain peak wavelengths.
 13. A semiconductorlaser device according to claim 12, wherein said resonator structureamplifies through selective feedback the light having a wavelengthdiffering from said plurality of the gain peak wavelengths as the lightof said specific wavelength.
 14. A semiconductor laser device accordingto claim 12, wherein said resonator structure feeds back selectively thelight having as said specific wavelength a wavelength lying intermediatebetween said different peak wavelengths.
 15. A semiconductor laserdevice according to claim 12, wherein said resonator structure feedsback the light having as said specific wavelength a wavelength otherthan those lying intermediate between said different peak wavelengths.16. A semiconductor laser device, comprising:a plurality of activeregions; means for injecting the carriers into said active regions; anda diffraction grating for feeding back light having a specificwavelength of those emitted from said active regions; wherein saiddiffraction grating is disposed in the vicinity of at least one of saidplural active regions, and wherein said plural regions inclusive of saidone region differ from one another in respect to change in the gainbrought about by the injection of carriers.
 17. A semiconductor laserdevice, comprising:a plurality of active regions; means for injectingthe carriers into said active regions; and a diffraction grating forfeeding back light having a specific wavelength of those emitted fromsaid active regions; wherein said diffraction grating is disposed in thevicinity of at least one of said plural active regions, and wherein saidplural regions inclusive of said one region differ from one another inrespect to change in the refractive index brought about by the injectionof carriers.
 18. A semiconductor laser device, comprising:a plurality ofactive regions; means for injecting the carriers into said activeregions; and a diffraction grating for feeding back light having aspecific wavelength of those emitted from said active regions; whereinsaid diffraction grating is disposed in the vicinity of at least one ofsaid plural active regions; and wherein said plural active layers aremade different from one another in respect to magnitude of change in thegain for an amount of carriers injected to thereby suppressself-oscillation only in a specific one of said plural active layers.19. A semiconductor laser device, comprising:a plurality of activelayers; means for injecting the carriers into said plural active layers;and a diffraction grating provided locally for oscillating light cf aspecific wavelength of those emitted by said active layers; wherein atleast one of said plural active layers is disposed in the vicinity ofsaid diffraction grating, whereby said plural active layers are renderedto differ from one another in respect to magnitude of change in therefractive index for an amount of carriers
 20. A semiconductor laserdevice, comprising:a plurality of active layers; means for injecting thecarriers into said plural active layers; and a diffraction gratingprovided locally for oscillating light of a specific wavelength of thoseemitted by said active layers; wherein change of refractive index/changein gain brought about by carrier injection in the first active layerdisposed in the vicinity of said diffractive index and/or in at leastone of the second one or more active layers are greater than those inthe other one(s) of said second active layers.
 21. A semiconductor laserdevice, comprising:a plurality of active layers; means for injecting thecarriers into said plural active layers; and a diffraction gratingprovided locally for oscillating light of a specific wavelength of thoseemitted by said active layers; wherein in the first active layerdisposed in the vicinity of said diffractive grating and in the secondone or more other active layer, either one or both of said first activelayer and said second active layer are formed in a multi-layerstructure, and wherein at least one of the active layers has a quantumwell layer whose thickness is smaller than that of said second activelayer.
 22. A semiconductor laser device according to claim 18, whereinsaid means for injecting the carriers are so provided as to be capableof injecting the carriers into said plurality of the active layersindependent of one another.
 23. A semiconductor laser device accordingto claim 18, including an optical waveguide for optically coupling saidplurality of the active regions to one another.
 24. A semiconductorlaser device according to claim 18, wherein said active layer providedin the vicinity of said diffraction grating is disposed in parallel withsaid grating, and wherein oscillation wavelength is changed by changingan amount of carriers injected into said active layer provided in thevicinity of said grating.
 25. A semiconductor laser device, comprising:aplurality of active layers; means for injecting carriers into saidplural active layers; and a diffraction grating provided locally on aregion through which light travels for oscillating light of a specificwavelength of those emitted by said active layers; wherein variations inrefractive index brought about by variations in the amount of injectedcarriers in said plurality of the active regions are mutuallycompensated for.
 26. A semiconductor laser device, comprising:aplurality of active layers; means for injecting carriers into saidplural active layers; and a diffraction grating provided on an opticalaxis along which light travels for oscillating light of a specificwavelength of those emitted by said active layers; wherein changes ingain brought about by injection of carriers in said plurality of theactive regions are different from one to another active regions.
 27. Asemiconductor laser device according to claim 25, wherein said pluralityof the active layers are disposed in cascade in the direction in whichlight travels.
 28. A semiconductor laser device according to claim 25,wherein said plurality of the active layers are disposed in parallel tothe direction in which light travels.
 29. A semiconductor laser deviceaccording to claim 25, wherein said plurality of the active layers haverespective semiconductor crystal compositions differing from oneanother.
 30. A semiconductor laser device according to claim 25, whereinat least one of said plural active layers has a quantum well.
 31. Asemiconductor laser device according to claim 25, wherein said pluralityof the active layers have quantum wells of different thicknesses.
 32. Asemiconductor laser device according to claim 25, including electrodesfor injecting the carriers into said plurality of the active layersindependent of one another.
 33. A semiconductor laser device, whereinthe optical path length of light traveling through a passivesemiconductor optical waveguide is changed by changing refractive indexof said optical waveguide by injecting carriers thereto, and wherein anoptical active layer is disposed in said optical waveguide in parallelwith the direction in which light travels through said optical waveguideto thereby reduce absorption loss produced in said optical waveguide bythe injection of the carriers.
 34. A semiconductor laser deviceaccording to claim 33, wherein gain of said optical waveguide is 30cm⁻¹.
 35. A semiconductor laser device according to claim 33, whereinthe layer thickness of the active material exhibiting maximum gain atthe wavelength of light guided through said optical waveguide is notgreater than 1/15 of the wavelength for use, and wherein the thicknessof the passive guide layer giving rise to absorption loss is not smallerthan 1/15 of the wavelength for use.
 36. A semiconductor laser deviceaccording to claim 33, wherein the wavelength at which said opticalactive layer exhibits a maximum gain is shorter than that of lightguided.